Pouring molten metal into a casting is a significant process variable that influences the internal soundness, surface conditions, and mechanical properties, such as tensile strength, porosity, percent elongation and hardness, of metal castings. Many different designs for dipping/pouring ladles exist and are used in the foundry industry. The designs are normally chosen based upon the type of molten material and casting mold used. Commonly used ladles make use of either a slot, lip and baffle, or a dam at the top of the ladle to reduce inclusion of furnace dross metal during metal filling, or they use a stopper rod to control the flow of metal into or out of the ladle.
Molten metals, such as aluminum, react with the air and instantaneously create oxides, commonly known as dross, which upon mixing with the rest of the molten metal creates inclusions and highly porous regions during solidification of the metal. While many factors influence and account for these undesirable properties, two common sources of inclusions include the dross layer formed on top of the molten metal, and the folding action of the molten metal caused by the vertical and horizontal momentum of the molten metal established during pouring.
Increased momentum gives rise to turbulent metal flow. Turbulent metal flow exposes more metal surface area to the air which creates the dross, or metallic oxide layer. Depending on the velocity of the molten metal, dictated by the pouring ladle and basin design and use, the molten metal may fold-over itself many times, thereby trapping these oxides and exposing still further surface area to the air.
Many current foundry ladles can be referred to as typical teapot-type ladles. These ladles are substantially cylindrically shaped with an external spout outwardly extending at the top. Certain teapot ladles have incorporated a wall, or baffle, which separates the large bowl or cavity area of the ladle from the spout and extends almost to the bottom of the ladle. When the molten material is poured, this baffle restricts the flow of molten metal to that which is near the bottom of the ladle, which is normally free from dross and any other foreign material that may be present, such as eroded refractory lining and ash from the fuel during the melting process. Although the baffle serves to minimize dross inclusion, the external spout design still increases the velocity of the material upon pouring, and may create turbulent flow.
Molten material is typically transferred from the ladle to the casting mold through a pour basin. In traditional pour basin designs, molten material flows down the basin to a mold sprue juxtaposed to a horizontal wall. Even traveling at a low velocity, if the molten material hits this square impact it often causes excessive turbulence in the molten material that leads to a folding over of the material, which in turn traps dross and metal oxides that are present on the molten material surface. This leads to inclusion of non-metallic particles into the casting that can reduce its mechanical properties. Non-conforming properties can lead to severe machinability problems and increase the propensity of undesired porosity during the subsequent solidification of the molten material.
Low pressure, bottom pour furnaces have been known to produce castings with minimal inclusion of dross and low porosity, but the equipment is costly, complex, and subject to high maintenance requirements. In addition, these furnaces require a significant capital cost requirement. Hot Isostatic Pressing (HIPping) of castings can also reduce porosity in aluminum castings, however this presently costs about $1 per pound to process the castings, and the castings cannot have any surface-connected porosity or the pores will not close. In addition, HIPping may cause dimensional changes to the casting that may not be uniform or repeatable.
Thus, there remains a need for an economical method and apparatus that would prevent or minimize the inclusion of dross and contaminants leading to high porosity and/or low mechanical properties of cast materials.